This invention relates to image processing, and more particularly relates to a novel optical image processor having particular application to the processing of X-ray images.
Image processors for medical X-ray images and the like are well known. Such image processors employ digital techniques to manipulate images.
Digital techniques are used in systems which employ digital computers for processing the received data. Typically, a video camera scans the output phosphor screen of an X-ray image intensifier and the output is digitized, and applied to a computer which performs varous manipulations on the data. Transparencies and other forms of input information can also be analyzed by the video camera.
Digital radiography equipment of this type is slow, is bandwidth-limited and has low spatial resolution and poor contrast detectability. Consequently, patients have to be exposed to fairly large doses of X-rays to employ this type of image processing.
Improved digital systems are available which have been developed principally for military reconnaissance applications and are characterized by moderate data through-put which might be equal to or greater than about 100 megabits per second and have the ability to implement advanced processing techniques including subtraction, convolution, correlation, etc. in real time or near real time. Even these digital signal processors have limitations, however, in that they are generally constrained to spatial resolutions of less than about 3 line pairs per millimeter. This is due to a combination of the limiting spatial resolution of the X-ray image intensifier and of the matrix size which is generally about 512.times.512 pixels. Further limitations exist on the quality of the end product image due to the television cameras used to image the output phosphor of the X-ray screen or image.
As the signal processing quality of digital signal processors is increased (that is, as the size of the matrix used is increased), the number of steps of gray to be manipulated is increased, the through-put is increased and the processing electronics becomes more complex. The equipment reaches a point where cost, maintenance, downtime, repair and environmental considerations become a serious problem in addition to the high initial cost of the equipment.
Optical image processing avoids many of the problems of digital signal processing at only a small fraction of the cost of the digital system. Conventional optical data processing equipment employs the properties and interactions which occur when light travels through a refracting medium. Various signal processing techniques can be used including convolution, correlation, image transformation, edge enhancement, deblurring and noise reduction among others.
Optical data processing systems generally employ operations on either coherent light or noncoherent light. The processing of coherent optical data streams offers a number of capabilities, principally the ability to perform a fourier transform in real time. This occurs when the coherent beam passes through a lens. Various types of spatial filters can be placed at the fourier plane of this lens. Such filters are capable of implementing various signal processing functions. The data stream is then subjected to an inverse fourier transform by a second lens. The resulting output image plane contains the enhanced data set.
Coherent optical data processing has certain limitations. These include a relatively small dynamic range and a noise problem known as "laser speckle" which is a snow-like noise image overlaid on the main image under certain circumstances. A second problem in coherent optical data processing is that the X-ray data which is initially input into the processor is incoherent so that appropriate transformation must be made prior to processing.
Noncoherent optical data processing is also known and is capable of performing noise reduction, feature enhancement, correlation, subtraction, level splicing among other data processing operations. Such processors offer a wide dynamic range and have a high signal-to-noise ratio with full two-dimensional parallel processing in real time of the data stream.
Noncoherent optical data processing techniques are based on two principles: geometric optical systems and diffraction optical systems. Each has its own advantages and disadvantages which are well known. Geometric optical systems employ shadow techniques which modify the optical data stream. Diffraction optics rely only on the interaction and diffraction of light waves to perform enhancement techniques.
The present invention provides a novel assemblage of optical components which integrates geometric optical techniques with diffraction techniques for a noncoherent optical data processor.